In somatic cells, mitosis does not occur before DNA synthesis is complete. However, this dependence can be disrupted by mutations or chemical treatments indicating that it is maintained normally by control mechanisms. These types of control mechanisms have been termed "checkpoints". (1). The checkpoint coupling mitosis to completion of DNA replication ensures that cells can survive periods of S-phase arrest resulting from exposure to inhibitors of DNA synthesis or DNA damage. As such inhibitors are frequently used in chemotherapy, it is important to understand how this control works. The specific aim of the experiments proposed here is to investigate this checkpoint using the fission yeast, Schizosaccharomyces pombe, as a model system. As the basic mechanisms controlling the cell cycle have been highly conserved during evolution, results obtained are likely to be applicable to all eukaryotic cells. Coupling of mitosis to completion of DNA replication requires an intracellular sensing pathway: proteins that control mitosis must "receive" information about chromosomes. Previous work has established that the highly conserved mitotic regulator cdc2 plays a central role in this process. Other proteins known to regulate activation of cdc2 are also involved. Recently, new genes (hus and rad genes) encoding proteins acting early in the pathway have been identified. these proteins are likely to be involved in "signalling" cdc2 that replication is incomplete. It is proposed to use the available mutants to determine what these "signals" are and how they are "detected" by cdc2 using a combination of molecular, genetic and biochemical techniques. In addition, it is proposed to identify novel gene functions that link these two processes.